1. Field of the Invention
The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same, and more specifically, to an electrolyte for use in a lithium-sulfur battery having excellent electrochemical properties such as battery capacity, high rate performance, cycle life, and energy density due to improved average discharge voltage.
2. Description of the Related Art
The development of portable electronic devices has led to a corresponding increase in the demand for secondary batteries having both a lighter weight and a higher capacity. To satisfy these demands, the most promising approach is a lithium-sulfur battery with a positive electrode made of sulfur-based compounds.
Lithium-sulfur batteries use sulfur-based compounds with sulfur-sulfur bonds as a positive active material, and a lithium metal or a carbon-based compound as a negative active material. The carbon-based compound is one which can reversibly intercalate or deintercalate metal ions, such as lithium ions. Upon discharging (i.e., electrochemical reduction), the sulfur-sulfur bonds are cleaved, resulting in a decrease in the oxidation number of sulfur (S). Upon recharging (i.e., electrochemical oxidation), the sulfur-sulfur bonds are re-formed, resulting in an increase in the oxidation number of the S. The electrical energy is stored in the battery as chemical energy during charging, and is converted back to electrical energy during discharging.
With respect to specific density, the lithium-sulfur battery is the most attractive among the currently developing batteries since lithium has a specific capacity of 3,830 mAh/g, and sulfur has a specific capacity of 1,675 mAh/g. Further, the sulfur-based compounds are less costly than other materials and are environmentally friendly.
Nevertheless, no lithium-sulfur batteries have yet been made widely commercially available . One reason these batteries have not been able to be commercialized is due to the poor sulfur utilization over repeated cycling, resulting in a low capacity. The sulfur utilization is referred to as a ratio of the amount of the sulfur involved in the electrochemical redox reaction of batteries to the amount of total injected sulfur. Further, the sulfur is diffused away to electrolytes upon the redox reaction, deteriorating the cycle life characteristics. Accordingly, unless the electrolyte is suitable, the reduced product of the sulfur, lithium sulfide (Li2S), is deposited, and as a result does not participate in further electrochemical reactions.
U.S. Pat. No. 6,030,720 describes liquid electrolyte solvents, including a main solvent having the general formula R1(CH2CH2O)nR2, where n ranges between 2 and 10, R1 and R2 are different or identical alkyl or alkoxy groups, and having a donor solvent of a donor number of 15 or more. Further, the main solvent includes a liquid electrolyte solvent that includes a solvent having at least one of a crown ether, a cryptand, and a donor solvent, which are solvents generating a catholyte after discharging. Despite using this kind of electrolyte, however, lithium-sulfur batteries have failed to obtain satisfactory capacity, high rate performance, or cycle life characteristics.
According to current research, an electrolyte of salts and an organic solvent may provide lithium ion batteries with a high ion conductivity and a high oxidation potential. In such lithium ion batteries, lithium salts such as LiClO4, LiBF4, or LiPF6 are mainly used. U.S. Pat. No. 5,827,602 describes non-aqueous batteries having lithium salts comprising triflate, imide, or methide-based anions. The aforementioned electrolyte shows good performance for lithium ion batteries. However, in lithium-sulfur batteries, the electrolyte causes problems by deteriorating the battery performance. This deterioration is due to the electrochemical reaction of the polysulfide being very unstable in a carbonate-based electrolyte, which is the most commonly used electrolyte in lithium-ion batteries. Thus, the lithium-sulfur batteries cannot effectively use the electrolyte present in the lithium-ion batteries. The electrolyte usable in lithium-sulfur batteries requires a stable electrochemical reaction with the polysulfide and requires the highly concentrated polysulfide generated by the reaction to be dissolvable. The characteristics of a lithium secondary battery depend on the kind and composition of salts and solvents. Up to now, the correct kind and composition of salts and solvents capable of improving cycle life and high rate characteristics of a lithium secondary battery, particularly a lithium-sulfur battery, have not been developed.